HCV has evolved remarkable mechanisms to ensure its persistence as a chronic infection. The mechanisms underlying the host innate and adaptive immune response to HCV infection, as well as the means by which HCV circumvents these measures to establish persistent infection, are not well characterized. Progress in understanding these mechanisms has been slowed by impediments in tissue culture and limitations in the ability to use authentic differentiated hepatocytes. However, recent advances in functional genomics and primary hepatocyte tissue culture methodologies now make identification of authentic host genes that contain HCV replication an attainable possibility. We hypothesize that the hepatocyte is a key locus for host control and containment of HCV infection. We propose the following Aims: 1. Define antiviral type IIFNstimulated genes in the hepatocyte and characterize their role in the establishment of chronicity. 2. Define the effects of HCV infection on the interaction of hepatocytes with the cellular immune response. 3. Define antiviral type II IFN-stimulated genes in the hepatocyte that participate in the noncytolytic clearance of HCV infection and characterize their role in the establishment of chronicity. To accomplish these Aims, we will utilize a high throughput whole genome siRNA screen we have successfully developed to identify genes that participate in HCV replication. We will use this screen to identify genes that participate in IFN-a and IFN-y's noncytolytic antiviral effects. We will exploit recent major advances in our ability to sustain primary human hepatocytes that both maintain function and support HCV infection. Primary hepatocytes from patient groups with variable susceptibility to chronic HCV will be studied for alterations in key antiviral IFN-stimulated genes identified in our functional genomic screens. The studies in this proposal are focused and thematically organized around the key role of the hepatocyte in the innate and adaptive immune responses, and their failure in chronic HCV. They have great potential to identify novel means of interrupting the viral lifecycle and preventing persistence of this model chronic infection.